Disc drive units in general are known in the art for use in modern microcomputers such as personal and desk top computers. In the most common form, such disc drive units comprise a so-called Winchester disc drive having one more rotatable memory storage discs mounted within a substantially sealed disc drive housing along with one or more related electromagnetic heads for reading and writing data on appropriately prepared disc surfaces. A disc drive of this type is sometimes referred to as a hard or a fixed disc drive and is normally available as a compact, complete package with the sealed housing on a rigid chassis or frame together with a circuit board carrying the necessary drive electronics.
Within the sealed disc drive, a rotating constant speed spindle motor is provided for mounting and rotating the discs. Because of the extremely fine tolerances in the internal components to the disc drive especially the head and the disc surface, the head disc assembly is sealed against the outside air to prevent the entry of contaminants. In order to minimize the size of the disc drive, modern spindle motors are designed with many components inside the hub which supports the discs. Therefore, it is also critical to provide a seal between the spindle motor which mounts the discs for rotation and the interior of the head disc assembly. Magnetic fluid seals are already widely used in spindle motors for disc drives as a means to seal portions of the regions within the head disc assembly to prevent the flow of air through the motor bearings or motor internal components, thus preventing particles such as grease aerosols from circulating from the motor into the interior of the head disc assembly.
It is standard practice in the disc drive industry in using magnetic fluid seals to use an adhesive to retain the seal in its defined location, and to prevent air flow around the outside diameter of the seal. A typical example of such a device shown in the simplest form appears in FIG. 1A and includes a housing 1 and shaft 3. A pair of annular pole pieces 13 and 15 are provided, sandwiching a permanent magnet 11 which is integrally inserted there between to form the ferro-fluidic seal of the device. The ferro-fluid 9 is disposed between the pole pieces 13 and 15 and the shaft 3. The magnet 11 causes the ferro-fluid to be retained in place between the pole-pieces 13 and 15 and the shaft 3 so that a seal is formed through which contaminants can not pass. As shown in this example, which is taken from U.S. Pat. No. 5,161,902 in order to prevent contaminants from passing through the outer radial gap b between the housing 1 and the pole pieces 13, 15, the gap between at least one of the pole pieces and the housing is filled with an adhesive bonding agent 23.
However, not only are ferro-fluid seals costly, but also are sensitive to packaging and shipping problems. Mishandling or sharp shocks to the disc drive or motor could lead to a splashed or burst seal, where the ferro-fluid migrates to the disc surfaces, leading to malfunction of the disc drive. A sharp pressure change could also lead to loss of fluid. Further, the use of adhesive could itself lead to contamination of the disc area.
Therefore, an alternative design which eliminates the need for the magnetic seal at the top of the shaft would be highly desirable. At the bottom of the shaft, adjacent the lower bearing, a clearance seal is typically provided, however, such a seal if utilized at the top of the shaft would require additional shaft length which is typically not available in a disc drive such as where these motor designs are commonly used.